Intensification of ozone gas/liquid mass transfer and ozonation efficiency: A critical review

Ozone (O₃) is effective for degrading persistent organic pollutants in wastewater but its efficiency is limited by poor O₃ gas/liquid mass transfer (k_La) and low O₃ utilization efficiency (typically 30 %-64 % in bubble columns). This review critically evaluates strategies to intensify O₃ mass transfer and enhance pollutant degradation. The k_La and energy consumption in different commonly used ozone contactors was compared, highlighting the importance of developing enhancement methods. Based on this, four types of techniques (i.e. applying physical fields, implementing membrane technology, utilizing micro/nano bubbles, and adding additives) were evaluated using k_La enhancement factor, pollutant degradation efficiency, and scalability as criteria. Physical methods (including ultrasound, electric field, and high gravity) enhanced k_La by 1.3-3 times but face scalability challenges due to high energy demands. Micro/nano-bubble producing systems coupled with catalysts such as activated carbon or chemical additives achieved k_La enhancements 3-4 times, increasing degradation of refractory pollutants by over 60 % removal. However, the microbubble generation also demands additional energy and chemical additives may cause secondary pollution. Natural mineral packings provide a balanced solution, enhancing k_La by 2.5-3 times and recalcitrant pollutant removal by 25-30 % at low energy consumption without secondary separation issues. For scalability, membrane contactors and catalytic microbubble have been applied at large-scale wastewater treatment while the stability of membrane and catalysts needs to be further improved. Overall, this study identifies favorable strategy for O₃ gas/liquid mass transfer and pollutant removal for sustainable water treatment.